I. Field of the Invention
The present invention relates to a motion-compensated interframe decoding apparatus of a television signal.
II. Description of the Prior Art
Conventionally, in a digital data television signal transmission system, the number of bits of transmission data is greatly decreased when an interframe coding system is employed wherein an interframe difference signal of two adjacent frames (i.e., frame difference signal) is coded and transmitted, instead of employing a conventional pulse code modulation (PCM) system. In particular, a still picture image or an image with little motion can be transmitted at a high compression rate (corresponding to a rate at which the number of bits of data to be transmitted is decreased as compared with that of the PCM system). However, in the case of transmitting data of a moving picture image, a large frame difference signals are generated, thereby decreasing the compression rate. In order to eliminate this drawback, "motion-compensated interframe coding" is proposed. According to this system, a motion of the object in the television signal is detected, and a prediction signal is obtained from a motion-compensated television signal, thereby performing predictive coding.
However, the conventional motion-compensated interframe coding system has the following drawback. When the frequency of sampling of the prediction error signal is large (i.e., when coarse quantization characteristics are used), picture quality is degraded to cause a so-called "dirty window" wherein a quantized signal noise component appears at the still image. Furthermore, the dirty window degrades a visual effect, such that the noise component is observed in a mosaic manner while in motion. In particular, this occurs at a still image or still portion thereof which has a small change in brightness (to be referred to as a still image portion as a whole hereinafter), resulting in inconvenience.